Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield the final desired toner image.
When heat-softenable toners, comprising, e.g., thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, at least one of which (usually referred to as a fuser roll) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll (usually referred to as a pressure roll) serves to press the receiver sheet into contact with the fuser roll.
The fuser roll usually comprises a rigid core covered with a resilient material, which will be referred to herein as a "base cushion layer." The resilient base cushion layer and the amount of pressure exerted by the pressure roll serve to establish the area of contact of the fuser roll with the toner-bearing surface of the receiver sheet as it passes through the nip of the pair of rolls. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roll. The degree of hardness (often referred to as "storage modulus") and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
Also, often the pressure roll and fuser roll have a regular cylindrical shape, but it has been found in the prior art to be advantageous in some cases to change the shape of the pressure roll in order to vary the amount of pressure exerted by the pressure roll against the receiver sheet and fuser roll. This variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established, for example, by continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roll a sort of "bow tie" or "hourglass" shape. This will cause the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip.
However, if, over time of use, the fuser roll begins to permanently deform to conform to the shape of the pressure roll, the gradient of pressure will be reduced or lost, along with its attendant benefits. It has been found that permanent deformation (alternatively referred to as "creep") of the base cushion layer of the fuser roll is the greatest contributor to this problem.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rolls is condensation-crosslinked poly(dimethylsiloxane) elastomer. "Poly(dimethylsiloxane)" will sometimes be alternatively referred to herein as "PDMS". The prior art has also taught or suggested that various fillers comprising inorganic particulate materials can be included in such PDMS base cushion layers to improve their mechanical strength and/or thermal conductivity. Higher thermal conductivity is advantageous when the fuser roll is heated by an internal heater, so that the heat can be efficiently and quickly transmitted toward the outer surface of the fuser roll and toward the toner on the receiver sheet it is intended to contact and fuse. Higher thermal conductivity is not so important when the roll is intended to be heated by an external heat source. Disclosure of such filled condensation-cured PDMS elastomers for fuser rolls can be found, for example, in U.S. Pat. Nos. 4,373,239; 4,430,406; and 4,518,655.
Optimal metal-filled elastomer fuser members have long been sought. At one time, it was predicted that:
"The metal of the metal-containing filler dispersed in the elastomer may be easily selected by one skilled in the art without undue experimentation by testing the metal-containing filler, such as a metal, metal alloy, metal oxide, metal salt or other metal compound, in an elastomer. The general classes of metals which are applicable to the present invention include those metals of Groups 1b, 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6b, 7b, 8 and the rare earth elements of the Periodic Table." (U.S. Pat. No. 4,264,181 to Lentz et al, column 10, lines 42-53; also U.S. Pat. No. 4,272,179 to Seanor, column 10, lines 45-54.)
This prediction of easy selection of the metal for a metal-containing filler has proven false in the face of latter efforts in the art.
A metal-containing filler which provides good results in one elastomer may provide very poor results in another elastomer, even if the elastomers are very similar. In U.S. Pat. No. 4,264,181 to Lentz et al, good results are obtained when lead oxide is used as a filler in various fluoroelastomers (Viton E430, Viton E60C, Viton GH; Examples X, XI, XII). In U.S. Pat. No. 5,017,432 to Eddy et al, on the other hand, the use of lead oxide in similar fluoroelastomers (for example, Viton GF) is taught against on the basis that it would produce an unacceptable fuser member. In these fluoroelastomers, cupric oxide is preferred. Similarly, U.S. Pat. No. 4,515,884 to Field et al, discloses a fuser member which utilizes metal oxide filled polydimethylsiloxane. The metal oxides are iron oxide and tabular alumina. Calcined alumina is described as being unsuitable per se (column 9, line 50--column 10 line 47).
An additional difficulty that has faced those attempting to produce metal-filled elastomer fuser members has recently been identified. In the past, it was thought that various materials' suitability for use in fuser roll base cushion layers in terms of their stability during use--i.e., their ability to resist degradation (as evidenced by weight loss), creep, and changes in hardness, during use in fuser rolls-could be determined by subjecting samples of the materials to conditions of continuous high temperature and continuous high stress (i.e., pressure), and then measuring the resultant changes in weight, shape (e.g., length), and hardness (e.g., storage modulus). However, J. J. Fitzgerald et al, "The Effect of Cyclic Stress on the Physical Properties of a Poly(Dimethylsiloxane) Elastomer", Polymer Engineering and Science, Vol. 32, No. 18, (Sept. 1992), pp. 1350-1357, indicates that such testing does not accurately portray the stability the materials will exhibit during actual use in fuser roll base cushion layers and that dynamic testing, with cycles of loading and unloading is necessary. The publication cites other reports showing the same kind of results in studies of other elastomers. Accordingly, the present inventors have helped to develop a device called a Mechanical Energy Resolver (sometimes alternatively referred to herein as an "MER"), which can be used to test samples of materials of interest for use in fuser roll base cushion layers. The device applies heat continuously to maintain the samples at a constant elevated temperature. The device also applies stress to the samples in the form of a compressive force, but does so in a manner such that the amount of compressive force applied varies cyclically (i.e., sinusoidally). The results of such testing consistently correlate with, and therefore reliably predict, the degree of stability a material will exhibit in the base cushion layer of a fuser roll during actual use.
The realization of the need for dynamic testing promises more accurate evaluation of filled elastomers, however, preparation of metal containing elastomers remains problematic. U.S. Pat. Nos. 4,515,884 to Field et al, and 5,017,432 to Eddy et al, cite large numbers of critical features or important aspects of their metal containing elastomers: choice of material (Field, column 9, lines 50-65 and column 10, lines 24-25), interaction of filler surface and elastomer (Field, column 9, lines 32-65), particle size (Field, column 10, lines 1-8 and lines 25-30; Eddy, column 9, line 65--column 10, line 3), concentration of metal-filler (Field, column 10, lines 9-23 and lines 31-47), capability of interacting with functional groups of release agent (Eddy, column 9, lines 26-30), reactivity of the metal filler with the elastomer (Eddy, column 9, lines 33-43), and acid-base characteristics of the metal filler (Eddy, column 9, lines 43-56). The lists of critical features and important aspects in Field and Eddy do not fully correlate. It is unknown whether this difference represents real differences in material characteristics or only differences in techniques and analysis.
One specific example of a condensation-crosslinked PDMS elastomer, which contains about 32-37 volume percent aluminum oxide filler and about 2-6 volume percent iron oxide filler, and which has been widely used and taught to be useful in fuser rolls, is sold under the trade name, EC4952, by the Emerson Cummings Co., U.S.A. However, it has been found that fuser rolls containing EC4952 cushion layers exhibit serious stability problems over time of use, i.e., significant degradation, creep, and changes in hardness, that greatly reduce their useful life. The present inventors have also found that MER test results correlate with and thus accurately predict the instability exhibited during actual use. Nevertheless, materials such as EC4952 initially provide very suitable resilience, hardness, and thermal conductivity for fuser roll cushion layers.
It would therefore be very desirable to be able to provide a fuser roll with a base cushion layer comprising a condensation-crosslinked PDMS elastomer containing appropriate fillers, wherein the cushion layer material will exhibit, under conditions of elevated temperature and cyclic stress, good stability, i.e., good resistance to degradative weight loss, creep, and changes in hardness. The present invention meets this need.